1. Field of the Invention
The present invention relates to a three-dimensional measurement method and a three-dimensional measurement apparatus for conducting measurement of a three-dimensional shape of an object surface using a non-contact three-dimensional measuring device such as an optical three-dimensional measuring device.
2. Description of the Related Art
Optical three-dimensional measuring devices have conventionally been used to conduct measurement of three-dimensional shapes of object surfaces for the purpose of visual inspection of the objects.
As the principles of general three-dimensional measuring devices, there are proposed a light section method using laser slit light or the like, a pattern projection method using pattern light, a stereoscopic method based on plural images obtained by image capturing of one object from plural different line-of-sight directions, a moiré method using moiré and others. The three-dimensional measuring devices obtain measurement data on an object using these measurement methods, perform three-dimensional calculation using the measurement data and various parameters and generate three-dimensional data (three-dimensional shape data) including three-dimensional coordinates (XYZ coordinates) and polygons of points on the object surface.
In the case of measurement with the three-dimensional measuring device, in order to obtain three-dimensional data of the entire surface of an object, it is necessary to conduct measurement with a position and posture of the three-dimensional measuring device variously changed for the object. To that end, the three-dimensional measuring device is held by a multi-joint manipulator such as a robot arm and measurement is conducted while the manipulator is controlled to change the relative position and posture between the object and the three-dimensional measuring device.
In such a case, the three-dimensional measuring device is held in various measurement postures such as an upward posture, a downward posture, sideways postures and a forward posture. On the assumption that the three-dimensional measuring device is used under such various measurement postures, it is so designed that a casing and important elements have sufficient rigidity to maintain measurement accuracy even if the measurement posture is changed. As shown in FIG. 7A, for example, in a three-dimensional measuring device using the light-section method, the rigidity of a light-projecting portion 20a, a light-receiving portion 20b or a support member SB for coupling the light-projecting portion to the light-receiving portion to support them is especially important.
In the case where the measurement posture of the three-dimensional measuring device is changed variously, the balance of center-of-gravity inside the three-dimensional measuring device is also changed and a bend or strain occurs marginally even if the casing or the elements of the three-dimensional measuring device has sufficient rigidity. When such a slight bend or strain occurs in the light-projecting portion, the light-receiving portion or the support member all of which affect the measurement accuracy of the three-dimensional measuring device, the positional relationship between the light-projecting portion and the light-receiving portion becomes out of balance as shown in FIG. 7B, and thereby measurement data is affected.
For example, the posture when the three-dimensional measuring device faces the front, i.e., the posture at forward zero degrees, is regarded as the reference. When the posture of the three-dimensional measuring device is set to upward or downward, the support member SB is expanded or contracted compared to the case of zero degrees, or a measurement space is distorted. This phenomenon is called a “posture difference”.
Incidentally, a user can conduct measurement of a three-dimensional object or the like to perform simple calibration in a three-dimensional measuring device in order to limit the influence of environmental changes such as temperature changes or changes with time upon the use of the three-dimensional measuring device. This is called “user calibrations”. However, a measurement posture when the user calibration is performed is a predetermined posture and is sometimes different from a measurement posture when measurement is actually conducted. In such a case, even if the user calibration is performed with high accuracy, a posture difference occurs due to the different postures, so that the measurement accuracy is reduced.
For the purpose of preventing such reduction in measurement accuracy based on the posture difference, there is conventionally proposed a method in which a detection sensor measures a bend amount or an elongation amount of a frame of a three-dimensional measuring device or others and calibration parameters for data correction are calculated based on data on the measured bend amount or elongation amount (U.S. Pat. No. 5,610,846).
According to the above-described conventional method in which a bend amount is measured, the actual bend amount or elongation amount is a slight amount of a few micrometers or less in many cases and detection with a detection sensor is difficult or high-accuracy detection is impossible. Thus, it is extremely hard to perform appropriate calibration by the conventional method.
Further, since the point where a bend or elongation is generated in a three-dimensional measuring device is not limited to one point, many detection sensors are required to measure all the bend amounts or all the elongation amounts accurately. This causes three-dimensional measuring devices to increase in size and cost.
Besides, since the relationship between bend amounts or elongation amounts in plural points and measurement errors is complicated in many cases, it is extremely difficult to accurately associate the bend amounts or elongation amounts with the measurement errors for each posture of a measuring device. For this reason, although much calibration data corresponding to each of the points and enormous calculation are required, proper correction with high accuracy cannot be expected.
Meanwhile, there is proposed a method in which mechanical rigidity of a casing or a support member of a three-dimensional measuring device is further improved, strain occurrence is prevented as much as possible and measurement errors are minimized. In such a case, however, a trade-off is inevitable in which the volume and the weight of the three-dimensional measuring device are increased and the material is changed to an expensive material having high rigidity.